Methods for forming an assembly for transfer of a useful layer

ABSTRACT

Methods for transferring of a useful layer from a support are described. In an embodiment, the method includes for facilitating transfer of a useful layer from a support by providing an interface in a first support to define a useful layer; and forming a peripheral recess on the first support below the interface so that the periphery of the interface is exposed to facilitate removal and transfer of the useful layer. An epitaxial layer can be formed on the useful layer after forming the recess, with the width and depth of the recess being sufficient to accommodate the volume of residual material resulting from formation of the epitaxial layer without covering the periphery of the interface. Alternatively, an epitaxial layer can be formed on the useful layer after forming the recess, wherein the peripheral recess is configured for receiving sufficient residual material from the epitaxial layer to prevent bonding between the residual material and the useful layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/800,252filed Mar. 11, 2004 and claims the benefit of provisional application60/490,796 filed Jul. 28, 2003. The content of each prior application isexpressly incorporated herein by reference thereto.

BACKGROUND ART

The present invention generally relates to methods of fabricatingsubstrates for microelectronics, optoelectronics, or optics involvingtransfer of a useful layer from a first support to a second support.

Various techniques have recently been developed to allow the mechanicaltransfer of a layer of semiconductor material from a first support to asecond support. The layer may or may not have undergone componentproduction treatments before the transfer. For example, one treatmentuses buried porous layers that can be attacked chemically and has beendescribed in U.S. Pat. No. 6,100,166. Another method utilizes substratesweakened by implanting gas species to obtain a thin useful layer thatcan be detached from the remainder of the material by fracture at theimplanted zone. Lastly, molecular bonding techniques utilize acontrolled bonding energy so that a mechanical force can result indetachment of a layer which has been temporarily bonded to a support.

When a useful layer is connected to a first support using one of theabove techniques, transfer of the layer involves bringing a secondsupport into contact with the free face of the useful layer usingsuitable bonding forces. The free face of the assembly including theuseful layer and the first support is known as the “front” face. Thetransfer is completed by applying stress forces (typically tensionand/or bending and/or shear forces) between the useful layer and thefirst support. One or more tools could be used to detach the usefullayer, such as a drawing rig or a blade introduced laterally at theweakened interface to propagate a crack, or a jet of fluid can beapplied to the weakened interface (see, for example, InternationalApplication Publication No. WO01/04933). When the useful layer has notundergone any component fabrication steps, then the transfer isgenerally carried out regardless of the bonding technique employed toaffix the useful layer to the second support (such as by molecularbonding, eutectic bonding, bonding using a polymer or resin, etc).

However, if the useful layer has already undergone steps in thecomponent fabrication process, then it is often necessary to carry outdifferent types of deposition treatments (for example, semiconducting orsemiconductive oxides or nitrides, polycrystalline semiconductor,amorphous semiconductor, monocrystalline semiconductor formed by homo-or hetero-epitaxy). For example, when a “full wafer” method is carriedout in a specific reactor, the deposits have a tendency to partially orcompletely cover the free face of the useful layer and to overflow ontothe side faces of the substrate that include the useful layer and thefirst support. Such an overflow produces a useful layer that isencapsulated resulting in a strengthening of the bond at the peripherybetween the useful layer and the first support. This condition can causeproblems during the subsequent detachment step required to transfer theuseful layer to the second support.

SUMMARY OF THE INVENTION

The present invention provides methods for forming an assembly fortransfer of a useful layer that overcomes the drawbacks mentioned above.In particular, in one embodiment, the method includes forming a usefullayer on a first support having an interface therebetween, and residualmaterial on a portion of the first support to form the assembly, andprocessing the assembly to attenuate bonding between the useful layerand the first support caused by the residual material.

In another embodiment, the invention relates to a method forfacilitating transfer of a useful layer from a support by providing aninterface in a first support to define a useful layer; and forming aperipheral recess on the first support below the interface so that theperiphery of the interface is exposed to facilitate removal and transferof the useful layer.

In an advantageous implementation, the useful layer is weakly bonded tothe support to facilitate detachment. In a variation, the interface maybe a detachable interface layer provided on the first support beforeforming the useful layer thereon.

This method can further comprise forming an epitaxial layer on theuseful layer after forming the recess, with the width and depth of therecess being sufficient to accommodate the volume of residual materialresulting from formation of the epitaxial layer without covering theperiphery of the interface. Alternatively, the method may furthercomprise forming an epitaxial layer on the useful layer after formingthe recess, wherein the peripheral recess is configured for receivingsufficient residual material from the epitaxial layer to prevent bondingbetween the residual material and the useful layer.

In another advantageous variation, processing of the assembly includesremoval of residual material. Generally, at least a portion of the firstsupport in contact with the residual material may be removed. Also, atleast a portion of a peripheral zone of material covering an edge of theinterface may be removed, with the removal made by at least one ofsplitting or etching. If desired, the useful layer may be masked beforeetching.

In another advantageous embodiment, processing of the assembly includesforming at least one cut or separating channel between a free surface ofthe useful layer and the interface to separate the useful layer fromcontact with the residual material. A plurality of separating channelsmay be cut to form a plurality of islets. The separating channel may becut by using at least one of a saw splitting technique, a lasersplitting technique, and an ion beam splitting and masked chemicaletching technique.

In yet another variation, processing of the assembly includes forming aperipheral recess in the first support so that the residual materialdoes not contact the useful layer. The width and depth of the peripheralrecess is sufficient to accommodate the volume of residual materialresulting from formation of the useful layer.

A further aspect of the invention pertains to a support for fabricatingsubstrates or components on substrates which can be treated to receiveat least a portion of a useful layer. The treatment results in theformation of a peripheral zone of material. The improvement comprisesproviding a peripheral recess zone in the support for receiving theresidual material to prevent bonding between the residual material andthe useful layer.

Another advantageous implementation includes a support for fabricatingsubstrates or components which can be treated to receive at least aportion of a useful layer. The treatment results in the formation of aperipheral zone of residual material, and thus a peripheral recessedzone is provided in the support for receiving the residual material toprevent bonding between the residual material and the useful layer.

Another advantageous embodiment of the invention concerns a substratethat includes a support and a useful layer having an interfacetherebetween. The useful layer is intended to be transferred to a secondsupport by affixing a free surface of the useful layer to the secondsupport and detaching it at the interface. The useful layer forms aperipheral zone of material that can at least partially cover theinterface, and the substrate includes at least one separating cut orchannel located between a free surface of the useful layer and theinterface to separate the useful layer from contact with the peripheralzone of material.

As described above, the present invention provides methods fortransferring a useful layer of a monocrystalline material from a firstsupport to a second support, a support and a substrate, for use infabricating substrates or components on substrates for microelectronics,optoelectronics, or optics. The method includes forming a firstsubstrate comprising the first support and at least a portion of theuseful layer with an interface between the first support and the usefullayer. A treatment involving the useful layer results in the formationof a peripheral zone of material that may at least partially laterallycover the interface. In an advantageous implementation, the material isremoved, thus enabling a detachment means to be used on the interface. Afree face of the useful layer is then affixed to a second support, andthe useful layer is detached at the interface between the first supportand the useful layer. Other advantageous methods are also describedherein for effectively transferring a useful layer from a first supportto a second support.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, purposes and advantages of the invention will becomeclear after reading the following description with reference to theattached drawings, in which:

FIG. 1 is a cross sectional view of a first substrate comprising a firstsupport and a useful layer;

FIG. 2 is a cross sectional view of a second support affixed to a firstsubstrate in order to transfer the useful layer thereto;

FIGS. 3 a and 3 b are cross sectional views illustrating removal ofresidual material according to this invention;

FIG. 4 is a cross sectional view of the first substrate illustrating aparticular arrangement of a first support according to the invention;and

FIGS. 5 and 6 are cross sectional and enlarged plan views respectively,of a particular arrangement of the useful layer for gaining access tothe interface between it and the first support according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a first support 10 that may be formed from asemiconductor material, for example silicon carbide SiC, mono- orpolycrystalline silicon, etc, or may be formed from an insulatingmaterial, for example sapphire. A layer 12 is formed or deposited on thefirst support and forms a releasable bonding interface. The layer 12 maybe a layer of semiconducting or semiconductive oxide such as SiO2,semiconductor nitride, or the like. The layer 12 forms a releasablebonding interface between the first support 10 and a useful layer. Inthis case, the useful layer includes a base layer 14 on which a layer 16has been formed or deposited. Typically, the base layer 14 is a seedlayer on which the layer 16 is formed by epitaxy. The seed layer may beformed of silicon carbide, sapphire, gallium nitride, silicon, oraluminum nitride, for example. In one embodiment, the base layer 14 isformed of SiC while the epitaxially grown layer is formed of a metalnitride such as gallium nitride GaN, or formed by a stack of differentmetal nitrides. Such a useful layer structure can advantageously be usedto fabricate light-emitting diodes (LEDs).

In a variation, the releasable bonding interface 12 between the firstsupport 10 and the useful layer may not be a layer as such. Instead, theuseful layer may be weakly bonded to the support 10 at the interfacetherebetween.

As shown in FIG. 1, the first support 10 is slightly larger than theassembly of layers 12, 14 and 16 formed on the support. The layer 16 maybe grown by epitaxy carried out in a “full wafer” reactor. Thus, thegrowth extends not only above the seed layer 14, but also around aperipheral ring 161 which covers the ledge or periphery of the support10.

FIG. 2 illustrates affixing the assembly shown in FIG. 1, termed thefirst substrate, to a second support 20. In this case, a metal bondingtechnique is used and the bonding layer is illustrated at 22. The usefullayer 14, 16 is transferred from the first support 10 to the secondsupport 20 after fixing or bonding as described above. A detachment toolmay be used to apply a stress or force to the interface layer 12 betweenthe useful layer 14, 16 and the first support 10, to detach the usefullayer by urging detachment in the plane of the interface.

However, FIG. 2 shows that the deposited GaN peripheral ring 161 causestwo problems. First, it reinforces the bond between the useful layer 14,16 and the first support at the periphery of the first substrate.Second, it hinders direct access to the bonding interface 12 by adetachment tool (such as a thin blade, a jet of fluid, etc.), and makesit impossible to directly apply the required detachment force or stressthereto (see arrow F1 in FIG. 3 a).

Several solutions for solving these problems are described below.

A first solution to this problem is shown in FIG. 3a. In thisimplementation, the peripheral ring 161 is removed. In a firstembodiment, removal can be achieved by etching. Thus, a mask may be usedon the free face of the useful layer 14, 16, to permit access only tothe ring 161. An attack medium suitable for removing the ring materialis then used to attack and remove the ring over its entire thickness,and thus to permit access to the detachable interface layer 12. In thepresent example, because the ring is made of GaN, the following ispreferably used: plasma etching or RIE (reactive ion etching) based onSiCl₄, BCl₃ (for further information, reference can be made to thearticle “GaN: Processing, Defects and Devices”, S. J. Pearton et al,Journal of Applied Physics, vol. 86, no 1, 1^(st) July 1999). Otheretching techniques such as plasma etching can be used.

In a second embodiment, the ring is removed using a splitting ortrimming technique. A mechanical saw, a laser or an ion beam could beused to split off or trim the peripheral ring 161 from the support 10.The ring may be removed by using a cylindrical cut and a cut in atransitional plane between the ring 161 and the first support 10. In allcases, care is taken that an attack or a splitting method providessatisfactory access to the detachable interface layer 12 to allowtransfer of the useful layer. It should be noted that merely partiallyremoving the ring 161 may be sufficient to attenuate the peripheral bondbetween the first support 10 and the useful layer 14, 16, and may beenough to allow proper use of a detachment tool. Alternately, it ispossible to remove the ring 161 while also penetrating into the supportitself.

FIG. 3 b illustrates another variation wherein splitting is carried outthrough the entire thickness of the first substrate so that not only theinterfering ring 161 is removed, but also removed is an adjacent portion101 of the first support 10. This method may be suitable when it isdifficult to control the working depth of the splitting technique. It ispreferable to remove the ring 161 before the second support 20 isaffixed to the useful layer 14, 16. However, in some cases it may bepossible to remove the peripheral ring after the second support 20 hasbeen bonded to the substrate.

FIG. 4 illustrates yet another approach for overcoming the problemcaused by the peripheral deposit 161. In this case, a first support 10is specifically prepared to include a peripheral recess 102. Theperipheral recess advantageously extends in a radial direction(horizontally in FIG. 4) between the outer edge of the support 10 andthe outer edges of the interface layer 12 and useful layer 14, 16. Inthe axial direction (vertically in FIG. 4), the recess 102 preferablyextends to a depth (d) that is at least equal to the thickness of theresidual material 161 deposited on the first support 10, so that at theend of the deposition operation, the peripheral ring 161 does notobstruct and/or does not bond to the detachable interface. Thus, in thiscase, the ring 161 does not have to be removed. The recess is preferablyproduced before forming the layers 12 and 14, and in any case beforeforming all or a portion of the useful layer which may cover theperiphery of the first substrate. Preferably, the recess is formed byremoving a portion of the first support with a laser beam or bymechanical trimming.

A further approach is shown in FIGS. 5 and 6. In this variation, cuts orchannels 18 are formed in the thickness of the useful layer 14, 16, to adepth equal to that of the interface layer 12. These cuts defineindividual islets or tiles 19. The islets may be square in shape, asshown in FIG. 6, with a size that is preferably in the range of from 1×1square micrometer (μm²) to 300×300 μm². The cuts can be formed eithermechanically, for example by using a saw splitting technique, or byusing a laser splitting or ion beam splitting technique. The cuts canalso be chemically etched by first placing an etching mask on the freesurface of the useful layer 14,16 that allows for a selective geometricattack. Preferably, and in particular to prevent the attack fromexcavating the walls of the channels too much, a dry or wet etchingtechnique is used. When the useful layer is formed from a seed layer ofSiC 14 on which GaN epitaxy is used, argon based ion etching may be used(for further information, see the article “GaN: Processing, Defects andDevices”, cited above).

Such an approach avoids reinforcement of the interface between theuseful layer and the first support caused by the presence of theperipheral ring 161. In particular, after producing the assembly shownin FIG. 1 channels are formed so that each individualized tile, which isnot itself subjected to the reinforcement due to the ring 161, can bedetached by force from the support. It should be noted that the forcecan be a tension, bending or shear stress, or a variety of combinationsof such forces.

Clearly, the present invention can be applied to a very wide variety ofsemiconductor materials. In addition to the example of a layer ofnitride developed on silicon carbide on insulator (SiCOI) as describedabove, the invention can be employed, for example, when transferring auseful silicon layer in which certain methods for fabricating componentsusing CMOS technology on a second insulating support 10 have beencarried out. Many other applications are also possible. The skilledperson will be able to readily select solutions that are suitable (i.e.choosing one of the three approaches described, the method of materialremoval, etc.) as a function of the materials used. In addition, thethree approaches of the invention can be combined together.

1. A method for facilitating transfer of a useful layer from a supportwhich method comprises: providing an interface in a first support todefine a useful layer; and forming a peripheral recess on the firstsupport below the interface so that the periphery of the interface isexposed to facilitate removal and transfer of the useful layer.
 2. Themethod of claim 1 wherein the useful layer is weakly bonded to thesupport at the interface to facilitate detachment.
 3. The method ofclaim 1 wherein the interface is a detachable interface layer providedon the first support before forming the useful layer thereon.
 4. Themethod of claim 1 which further comprises: affixing a free face of theuseful layer to a second support; and detaching the useful layer at theinterface to transfer the useful layer to the second support.
 5. Themethod of claim 4 which further comprises detaching the useful layer byusing at least one of tension forces, bending forces and shear stressforces.
 6. The method of claim 4 which further comprises directing atleast one of a thin blade or a jet of fluid to the interface layer todetach the useful layer.
 7. The method of claim 1 which furthercomprises forming an epitaxial layer on the useful layer prior toforming the recess.
 8. The method of claim 7 wherein residual materialdeposited with the epitaxial layer is removed when forming theperipheral recess.
 9. The method of claim 8 which further comprisesremoving at least a portion of the first support that is in contact withthe residual material.
 10. The method of claim 8 wherein the residualmaterial is removed by at least one of splitting or etching.
 11. Themethod of claim 10 wherein the peripheral zone is removed by etching andwhich further comprises masking the useful layer prior to etching. 12.The method of claim 1 which further comprises forming an epitaxial layeron the useful layer after forming the recess, with the width and depthof the recess being sufficient to accommodate the volume of residualmaterial resulting from formation of the epitaxial layer withoutcovering the periphery of the interface.
 13. The method of claim 1 whichfurther comprises forming an epitaxial layer on the useful layer afterforming the recess, wherein the peripheral recess is configured forreceiving sufficient residual material from the epitaxial layer toprevent bonding between the residual material and the useful layer. 14.The method of claim 1 which further comprises using full wafer epitaxyto deposit an epitaxial layer on at least a portion of the useful layer.15. The method of claim 1 wherein the useful layer comprises a seedlayer for epitaxial growth and at least one epitaxial layer.
 16. Themethod of claim 15 wherein the seed layer is made of at least one ofsilicon carbide, sapphire, gallium nitride, silicon and aluminumnitride.
 17. The method of claim 15 wherein the epitaxial layer isformed from one or more metal nitrides.
 18. The method of claim 1wherein the first support is made from at least one of a semiconductor,a semiconducting or semiconductive carbide, and an insulator material.19. The method of claim 1 which further comprises providing theinterface by at least one of implanting gas species, forming a porouslayer that can be attacked chemically, or bonding a detachable layer tothe first support before forming the useful layer by using a controlledmolecular bonding process.